Abstract: Objective To develop a simple and sensitive method for simultaneous determination of chlorpyrifosO,O-diethyl-O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate,CPF and its specific metabolite 3,5,6-trichloro-2-pyridinol (TCP) in plasma by GC. Methods Plasma of rabbit (0.50 mL) was extracted by ethylacetate. After purification and centrifugation,the organic layer was evaporated with a stream of nitrogen at 40℃. The residue was dissolved in ethylacetate, and TCP was derivatized for an hour at 50℃ with N-(tert-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA). Then the sample was injected into the GC system with HP-5 column and ECD. Qualitative analysis was made by the retention time and quantitative analysis by external standard curve. Results Precise separation of CPF and TCP was achieved within 8 minutes. The relationship between peak areas and concentration showed a good linearity in a range from 0.01 to 2.50 mg/L for both CPF and TCP (r=0.999 7, 0.999 8, respectively). The limits of detection were 1μg/L for both CPF and TCP. Percentage recoveries of this procedure in different matrix were between 77.60%-106.16% for CPF, and between 72.00%-99.86% for TCP. The RSDs of the variation were both lower than 10%. Conclusion This proposed method is simple,reliable and sensitive for simultaneous determination of CPF and its metabolite TCP in plasma samples with satisfactory results.